The invention relates to a system for transmitting coded speech signals employing a very complex coding as described, for example, in the title "Advances in Speech Coding" by Bishnu and S. Atal, Vladimir Cuperman and Allen Gersho, Kluwer Academic Publishers 1991, more specifically, pp. 69 to 79 and a mobile station for such system and a circuit arrangement for such mobile station. A system of this kind is used especially for mobile radio to transmit speech signals between a mobile station and a fixed station. Such a coding requires much circuitry which can be realised in practice only with a highly sophisticated signal processor.
The speech signals are generated by natural speakers. As a result, during a conversation there are always speech pauses in which one side does not speak, but hears only the speech signals from the other side. To give the speaking person on this other side the certainty, however, that the connection between the two sides still exists, although the person on the first side does not speak for the moment, this first side generates codes for background noise and transmits same.
For the utilization of a speech link, the people speaking normally utilize a handset in which the receiver, i.e. loudspeaker and the microphone, are spatially separated, so that normally only a slight part of the acoustic signal produced by the loudspeaker reaches the microphone of that same handset. As a result, each speaker receives only a weak echo which is additionally slightly delayed and thus does not have an annoying effect. However, it is also desirable to utilize hands-free facilities in which the microphone and the loudspeaker both have a certain distance from the speaking person and in which there is also the effect that an acoustic signal transmitted by the loudspeaker reaches the microphone with a considerable amplitude and delay, so that the other side of the line receives a strong and considerably delayed echo, which is very annoying. To avoid this, cancelling arrangements are known to be used for cancelling acoustic echoes. They comprise a subtracter arrangement, a filter for filtering received speech signals and an arithmetic unit for calculating the filter coefficients of the filter. The filtered received speech signals are applied to the subtracter arrangement, and the arithmetic unit sets the filter coefficients, so that at the output of the subtracter arrangement the acoustic signal captured from the microphone is just compensated for by the filtered signal. The arithmetic unit calculates the filter coefficients only during a period of time in which nothing is spoken at the location in which echo cancellation takes place. For a proper echo cancellation, however, it is necessary to have an expensive arithmetic unit. Additionally, it is also possible to have controlled attenuations in the signal paths, as is described in the journal "Philips Telecommunication Review", Vol. 49, No. 1, March 1991, pp. 19 to 27. Due to this required expenditure, it is hard to realise a hands-free facility for speech signal transmission with complex coding, because the computational capability of a signal processor is normally insufficient and a 2-processor solution will be too costly.